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The use of fuels made from crops, waste, algae and other biological materials to power aircraft isn’t ready for takeoff.

These kinds of alternative fuels are the best chance for aviation to cut its greenhouse gas emissions in the coming decades. That’s a crucial step in curbing global warming, because aviation accounts for at least 2 percent of worldwide CO2 emissions and is on track to take a much bigger share as other industries shrink their carbon footprints.

Yet the industry isn’t likely to adapt to them for several decades without more of a nudge.

Biofuels for aviation are subject to much tighter rules than for any other form of transport, for example to make sure they don’t freeze in the air. Prices are hard to peg because jet fuel is sold under commercial contracts, but these fuels are estimated to cost two to three times more than biofuels for cars and two to seven times more than fossil fuels, according to industry sources.

Plus, airplanes lack the alternatives available for road and sea transport like renewable electricity, hydrogen or lower-emitting natural gas, which aren’t dense enough to provide the huge amounts of energy needed to take off and land. Electric batteries are still too heavy to use in aircraft.

“At least in the medium term, probably 20 to 30 years, we don’t see a viable option apart from using a liquid hydrocarbon fuel,” said Robert Boyd, the International Air Transport Association’s manager for environment and alternative fuels.

In the European Union, the tussle over incentives for clean transport fuels hinges on updates to the bloc’s renewable energy policy for 2020 to 2030, now under negotiation.

And even shifting to more biofuels won’t happen without incentives to spur research and development and help bring down the cost, he said. His industry group says sustainable alternative fuels are already technically ready to use, and could cut CO2 by up to 80 percent, but governments need to step in and make them commercially viable.

Airborne biofuels

In the European Union, the tussle over incentives for clean transport fuels hinges on updates to the bloc’s renewable energy policy for 2020 to 2030, now under negotiation.

Biofuels are a highly divisive issue. Environmentalists argue they’re not all that green because they can release carbon dioxide stored in plants and cut into food supply. The industry counters that the plants will grow back and suck the CO2 back from the atmosphere, that the crops give farmers a new source of demand, and that organic fuel is better than traditional fuel.

The role for first generation biofuels, made from food crops, is now being limited. Instead, the new renewables policy is shifting to tightening the sustainability criteria for advanced versions, which use woody crops, agricultural residue, waste and other organic materials to produce fuel.

The European Commission’s proposal last November tries to encourage fuel suppliers to point their advanced, more sustainable biofuels toward aviation and shipping.

Overall, fuel suppliers would have to increase the amount of clean fuel in their mix across the transport sector to 6.8 percent by 2030. That includes many lower-emitting alternatives to oil, but at least 3.6 percent should come from advanced biofuels. But here’s the bonus for aviation and shipping: Every unit of biofuel that goes toward an aircraft or ship would be would be worth 1.2 units, making it an attractive way to meet the target.

“I would give a much bigger multiplier for aviation than for shipping, because otherwise the shipping industry would easily get the fuel” — German MEP Peter Liese

The aviation industry likes this idea, but argues it’s not enough. Not only does the sector have fewer options for clean energy sources, but its strict safety standards limit the types of biofuels it can use.

The answer, according to aviation and biofuels industry sources, is to give aviation an even greater incentive to account for its more limited options compared to the maritime industry — around 2, according to one source. European aircraft manufacturer Airbus said it’s hard to predict how the market will develop, so the rules should be reassessed every few years.

“[Maritime has] access to other types of technologies, they have no weight limitations, they are currently using the cheapest fuel on the market — crude oil, whereas we are using a very regulated fuel — so it’s absolutely not comparable,” the aviation industry source said, asking not to be named. “We share the same ambition, but we don’t share the same technological options.”

The European shipping industry agrees that aviation has fewer options, and isn’t contesting the bid for more incentives. What shipping wants is a policy that encourages all types of clean alternatives, not just biofuels.

Blame it on the plane

Dutch Green MEP Bas Eickhout, who leads the European Parliament environment committee’s discussion on renewable energy rules, sees the need to give fuel suppliers a bigger nudge toward aviation — but not by giving the industry a separate and higher multiplier.

Earlier this month Eickhout proposed deleting the 1.2 multiplier for two reasons. It wouldn’t be enough to make up for the higher cost of aviation fuels, and shipping doesn’t need biofuels to cut emissions, it needs slower speeds and wind and solar energy.

Instead, he wants to apply the Commission’s proposed 6.8 percent target for green fuels across transport specifically to aviation as well, where advanced biofuels won’t face competition from the other sources available to road and maritime transport.

“The market will move to aviation because there’s no competition from electricity,” Eickhout said. “Advanced applications will move toward aviation, and that’s exactly what I want.”

Chris Goater, assistant director of corporate communications at the International Air Transport Association, said the principles underpinning Eickhout’s proposals are “interesting,” but the association isn’t convinced. “In its current form the proposal lacks sufficient detail to enable the industry to support it at this stage,” the association said in an emailed statement.

The center-right European People’s Party (EPP) is likely to side with the aviation industry’s demands.

“You have to think more about using waste streams and algae and things like that, but that’s the future, we’re not there yet” — Dutch MEP Bas Eickhout

“I would give a much bigger multiplier for aviation than for shipping, because otherwise the shipping industry would easily get the fuel,” said German MEP Peter Liese, the EPP’s representative in the talks. “If member states have a cheap option, they will not invest in aviation.”

The NGO Transport & Environment welcomed Eickhout’s proposal to get rid of the multiplier, but cautioned that benefits for jet fuels risk becoming an extra indirect cost for road drivers. As long as international aviation and shipping get “preferential treatment” by, for instance, not paying fuel taxes, “there shouldn’t be any incentives for the use of advanced fuels in aviation,” said Laura Buffet, the group’s clean fuels manager.

Green groups largely acknowledge that advanced biofuels are needed — but only as part of a broader effort that also aims to increase engine efficiency, impose strict sustainability criteria and a higher price for trading emissions, among other measures, according to Transport & Environment.

But biofuels shouldn’t be the final goal for green aviation, they argue. The Solar Impulse 2 single-seat airplane flight around the world last year showed that aviation could one day shift to solar panels and other renewables. However, that remains a far-off dream for commercial passenger jets.

For now, the industry still needs to focus on improving biofuel’s green credentials, Eickhout said.

“You have to think more about using waste streams and algae and things like that, but that’s the future, we’re not there yet,” he said. “That’s why I’m skeptical now. At this moment it’s really for show.”